The invention pertains to electron guns as used in apparatus and methods utilizing an electron beam, especially apparatus and methods in which an electron beam is used to perform projection of an image of a pattern (such as an integrated circuit pattern), as defined on a reticle, to a sensitive substrate (such as a semiconductor wafer). The invention also pertains to apparatus including such guns and to methods for manufacturing devices (e.g., semiconductor integrated circuits), wherein the methods utilize such projection apparatus.
A key technology in manufacturing integrated circuits and displays is microlithography (image-transfer and imprinting technology). Feature sizes and line widths of integrated circuits progressively are becoming more miniaturized and have now reached the resolution limit of light (visible and ultraviolet light as used in xe2x80x9copticalxe2x80x9d microlithography). Electron-beam microlithography currently is under intensive investigation as a possible successor to optical microlithography, especially in view of the potentially greater resolving power of electron-beam microlithography compared to optical microlithography.
In electron-beam microlithography, an electron beam is produced by an electron gun. The beam is directed to a reticle (sometimes termed a xe2x80x9cmaskxe2x80x9d) that defines the pattern to be transferred. The beam illuminates the pattern, or a selected portion thereof on the reticle, and the portion of the beam passing through the illuminated portion of the reticle is directed to a selected region of the substrate. More specifically, the electron beam propagating from the electron gun to the reticle is termed the xe2x80x9cillumination beam,xe2x80x9d which passes through an xe2x80x9cillumination-optical systemxe2x80x9d to the reticle. The illumination-optical system typically includes multiple electromagnetic lenses that converge the illumination beam appropriately for illuminating the desired region of the reticle. Upon passing through the reticle, the illumination beam acquires an ability to form an image of the illuminated portion of the reticle; thus, the beam propagating downstream of the reticle is termed the xe2x80x9cpatterned beam.xe2x80x9d The patterned beam passes through a xe2x80x9cprojection-optical systemxe2x80x9d to the substrate. The projection-optical system typically includes a pair of electromagnetic projection lenses that form a focused image, of the illuminated portion of the reticle, of a desired size on a corresponding region of the substrate. Hence, the image defined by the reticle is projected onto the substrate, usually portion-by-portion. This general process is also termed xe2x80x9cpattern transferxe2x80x9d because the pattern defined by the reticle effectively is xe2x80x9ctransferredxe2x80x9d to the substrate.
Conventional microlithography apparatus as summarized above normally produce a xe2x80x9cdemagnifiedxe2x80x9d (or xe2x80x9creducedxe2x80x9d) image on the substrate. This means that the image as formed on the substrate is smaller, usually by an integer factor, than the corresponding illuminated region on the reticle. The reciprocal of the integer factor is termed the xe2x80x9cdemagnification ratio,xe2x80x9d of which a representative value is 1/4 or 1/5.
Electron guns used in conventional electron-beam microlithography apparatus of the type summarized above generally include three electrodes. The first electrode is a cathode used within a temperature-limitation region of its intensity-temperature (I-T) profile (FIG. 3). The second electrode is an anode that is charged appropriately to pull electrons away from the cathode to propagate through an axial aperture defined by the anode. The third electrode is a Wehnelt electrode (also termed a xe2x80x9cWehnelt cylinderxe2x80x9d) that serves, inter alia, to guide electrons from the cathode through the anode aperture and thus, by preventing impingement of the electrons on the anode, reduce heating of the anode. In this conventional electron-gun configuration, the cathode and Wehnelt electrode are insulated electrically from each other, and have different electrical potentials (voltages) applied to them.
Many types of conventional electron-beam microlithography systems (e.g., variable-shaped pattern systems, character-projection systems, and divided-pattern projection systems) utilize a xe2x80x9csolidxe2x80x9d electron beam having a transverse profile (e.g., gaussian or rectangular) in which the beam intensity at the contrast aperture is greatest at the center of the beam. However, it has been found that, in such systems, a solid beam is subject to xe2x80x9cspace-charge effectsxe2x80x9d that are manifest as, e.g., focal-point shift, increase in beam blur, and distortion of the pattern as projected onto a wafer or other suitable substrate. In effort to solve problems associated with space-charge effects, electron guns have been investigated that produce a xe2x80x9chollow beamxe2x80x9d in which, at the contrast aperture, the most intense portion of the beam is not located at the center of the beam, but rather at peripheral regions of the beam.
Unfortunately, no effective methods or apparatus exist to date for evaluating or adjusting a hollow beam.
As shown in FIG. 3, within the temperature-limitation region of the cathodic I-T profile, even a slight change in cathode temperature causes a substantial change in the intensity of beam current produced by the cathode. Consequently, in an electron gun in which the cathode is operated under temperature-limitation conditions, any irregularity in cathode-surface temperature or irregularity in the work function of the cathode surface causes the intensity distribution of the electron beam to be not uniform. An electron beam produced under such conditions does not provide a desired uniform illumination of the reticle. As a result, the dimensional accuracy of the pattern as transferred onto the substrate is degraded. This problem is difficult especially whenever a hollow beam is used.
For example, in order for an electron-beam microlithography apparatus to be viable commercially for high-volume production, it must have a per-shot exposure area of at least 1 mmxc3x971 mm at the reticle, which requires a cathodic surface having an area of 3 to 10 mm2. Variations arising from the cathode being operated under temperature-limitation conditions can be substantial, especially with a gun having a cathode configured to produce a hollow beam.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide electron guns, for use in electron-beam microlithography apparatus that produce a demagnified image of the reticle pattern on the substrate, in which the transverse distribution of electron-beam intensity can be made uniform or otherwise adjusted as required.
To such end, and according to a first aspect of the invention, electron guns are provided that comprise a cathode, an anode, and a filament array. The cathode comprises an electron-emitting surface that emits a beam of electrons whenever the cathode is energized electrically. The anode is situated downstream of the cathode and can be energized at a voltage appropriate for drawing electrons from the cathode. The filament array is situated adjacent the cathode (e.g., adjacent an upstream-facing surface of the cathode) and is configured to energize respective regions of the cathode in a selective manner. The filament array comprises multiple filaments that are controllable independently to allow independent adjustment of electrical energy from the filaments to respective regions of the cathode.
Typically, the cathode and anode are arranged on an axis (xe2x80x9coptical axisxe2x80x9d), and the multiple filaments are arranged equidistantly from one another radially around the axis. For example, the filament array can comprise eight independently controllable filaments. In a particularly advantageous configuration, the electron-emitting surface is ring-shaped about the axis so as to emit a hollow beam of electrons, where each filament is adjacent a respective region of the ring-shaped electron-emitting surface.
The electron gun can include a control anode situated between the cathode and the anode. The electron gun also can include a Wehnelt electrode.
Each filament in the filament array is connected typically to a respective power supply and a respective bombardment-voltage power supply. The power supplies and bombardment-voltage power supplies are connected desirably to a CPU interface (or analogous controller) configured to energize the filaments and bombardment-voltage power supplies independently. For example, the filament array can be configured to bombard, when energized by the bombardment-voltage supplies, electrons onto an upstream-facing surface of the cathode. In such a configuration, each bombardment-voltage supply is controllable independently to allow independent adjustment of respective currents of electrons from the filaments bombarding the upstream-facing surface of the cathode.
The CPU interface can be connected to a computer or the like that is configured to receive and process data concerning a transverse beam-intensity profile of the electron beam and to route control signals to the bombardment-voltage power supplies as required to change the transverse beam-intensity profile of the electron beam. Alternatively or in addition, the CPU interface can be connected to a display. The display is configured to display data concerning a transverse beam-intensity profile of the electron beam. Such data can be used by an operator who inputs control commands to the CPU interface appropriate for causing the CPU interface to route control signals to the bombardment-voltage power supplies as required to change the transverse beam-intensity profile of the electron beam.
According to another aspect of the invention, electron-beam optical systems are provided, especially for use in an electron-beam microlithography apparatus. A representative embodiment of such a system comprises, on an optical axis, an electron gun, an illumination-optical system, and a projection-optical system.
The electron gun of the system comprises a cathode comprising an electron-emitting surface that emits an illumination beam of electrons whenever the cathode is energized electrically. The electron gun also comprises an anode situated downstream of the cathode. The anode can be energized at a voltage appropriate for drawing electrons from the cathode.
The electron gun can include a filament array situated adjacent the cathode and configured to energize respective regions of the cathode in a selective manner. The filament array comprises multiple filaments that are independently controllable to allow independent adjustment of electron current from the filaments to the respective regions of the cathode.
The illumination-optical system is situated downstream of the electron gun and is configured to direct the illumination beam to a region on a reticle situated downstream of the illumination-optical system. The region is illuminated by the illumination beam so as to produce a patterned beam propagating downstream of the reticle. The projection-optical system is situated downstream of the reticle and is configured to direct the patterned beam to a region on a substrate so as to imprint the substrate with a pattern defined on the reticle.
The system also comprises a first aperture situated off-axis, a first deflector, and a first detector. The first deflector is situated and configured to deflect, whenever the first deflector is energized, either the illumination beam or the patterned beam to the first aperture and to scan the beam relative to the first off-axis aperture. The first detector is situated relative to the first aperture and configured to obtain data concerning a transverse beam-intensity profile as the beam is scanned relative to the first off-axis aperture.
The first off-axis aperture and first deflector can be situated in the illumination-optical system in which the first deflector deflects and scans the illumination beam relative to the first aperture. A system having such a configuration also can comprise a second off-axis aperture, a second deflector, and a second detector. The second off-axis aperture is situated off-axis in the projection-optical system. The second deflector is situated in the projection-optical system and configured to deflect, whenever the second deflector is energized, the patterned beam to the second off-axis aperture and to scan the patterned beam relative to the second off-axis aperture. The second detector is situated relative to the second off-axis aperture and is configured to obtain data concerning a transverse beam-intensity profile as the patterned beam is scanned relative to the second off-axis aperture.
Alternatively, the first off-axis aperture, first deflector, and first detector can be situated in the projection-optical system. In such a configuration, the first deflector deflects and scans the patterned beam relative to the first off-axis aperture.
According to another aspect of the invention, methods are provided for detecting and adjusting a transverse beam-intensity profile of an electron beam produced in an electron-beam microlithography apparatus. (The apparatus includes, along an optical axis, an electron gun that produces an electron beam, an illumination-optical system that directs the electron beam to a reticle, and a projection-optical system that receives the electron beam from the reticle and directs the beam to a substrate.) In a representative embodiment of the method, the electron gun is provided with multiple filaments adjacent a cathode of the electron gun. Each filament is connected to a respective power supply and a respective bombardment-voltage supply, and each filament can be energized selectively to adjust an output of electrons from a respective region of the cathode. An off-axis aperture is provided on a plane at a position at which either the illumination-optical system or the projection-optical system forms an image of a beam crossover (e.g., gun crossover). A detector is situated downstream of the off-axis aperture and a deflector is situated upstream of the off-axis aperture. The deflector is energized to cause the deflector to deflect the electron beam laterally to the off-axis aperture. The electron beam is scanned across the off-axis aperture. Using the detector, electrons are detected that have passed through the off-axis aperture so as to produce a data signal corresponding to a transverse beam-intensity profile of the electron beam. Based on data in the data signal, electrical energy provided to at least some of the power supplies and bombardment-voltage supplies can be adjusted selectively as required to cause a change to the transverse beam-intensity profile.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.